Lab: xv6 lazy page allocation

One of the many neat tricks an O/S can play with page table hardware is lazy allocation of user-space heap memory. Xv6 applications ask the kernel for heap memory using the sbrk() system call. In the kernel we've given you, sbrk() allocates physical memory and maps it into the process's virtual address space. It can take a long time for a kernel to allocate and map memory for a large request. Consider, for example, that a gigabyte consists of 262,144 4096-byte pages; that's a huge number of allocations even if each is individually cheap. In addition, some programs allocate more memory than they actually use (e.g., to implement sparse arrays), or allocate memory well in advance of use. To allow sbrk() to complete more quickly in these cases, sophisticated kernels allocate user memory lazily. That is, sbrk() doesn't allocate physical memory, but just remembers which user addresses are allocated and marks those addresses as invalid in the user page table. When the process first tries to use any given page of lazily-allocated memory, the CPU generates a page fault, which the kernel handles by allocating physical memory, zeroing it, and mapping it. You'll add this lazy allocation feature to xv6 in this lab.

Before you start coding, read Chapter 4 (in particular 4.6) of the xv6 book, and related files you are likely to modify:

To start the lab, switch to the lazy branch:

  $ git fetch
  $ git checkout lazy
  $ make clean

Eliminate allocation from sbrk()

Your first task is to delete page allocation from the sbrk(n) system call implementation, which is the function sys_sbrk() in sysproc.c. The sbrk(n) system call grows the process's memory size by n bytes, and then returns the start of the newly allocated region (i.e., the old size). Your new sbrk(n) should just increment the process's size (myproc()->sz) by n and return the old size. It should not allocate memory -- so you should delete the call to growproc() (but you still need to increase the process's size!).

Try to guess what the result of this modification will be: what will break?

Make this modification, boot xv6, and type echo hi to the shell. You should see something like this:

init: starting sh
$ echo hi
usertrap(): unexpected scause 0x000000000000000f pid=3
            sepc=0x0000000000001258 stval=0x0000000000004008
va=0x0000000000004000 pte=0x0000000000000000
panic: uvmunmap: not mapped
The "usertrap(): ..." message is from the user trap handler in trap.c; it has caught an exception that it does not know how to handle. Make sure you understand why this page fault occurs. The "stval=0x0..04008" indicates that the virtual address that caused the page fault is 0x4008.

Lazy allocation

Modify the code in trap.c to respond to a page fault from user space by mapping a newly-allocated page of physical memory at the faulting address, and then returning back to user space to let the process continue executing. You should add your code just before the printf call that produced the "usertrap(): ..." message. Modify whatever other xv6 kernel code you need to in order to get echo hi to work.
Here are some hints:

If all goes well, your lazy allocation code should result in echo hi working. You should get at least one page fault (and thus lazy allocation), and perhaps two.

Lazytests and Usertests

We've supplied you with lazytests, an xv6 user program that tests some specific situations that may stress your lazy memory allocator. Modify your kernel code so that all of both lazytests and usertests pass.

Your solution is acceptable if your kernel passes lazytests and usertests:

$  lazytests
lazytests starting
running test lazy alloc
test lazy alloc: OK
running test lazy unmap...
usertrap(): ...
test lazy unmap: OK
running test out of memory
usertrap(): ...
test out of memory: OK
$ usertests

Submit the lab

This completes the lab. Make sure you pass all of the make grade tests. If this lab had questions, don't forget to write up your answers to the questions in answers-lab-name.txt. Commit your changes (including adding answers-lab-name.txt) and type make handin in the lab directory to hand in your lab.

Time spent

Create a new file, time.txt, and put in it a single integer, the number of hours you spent on the lab. Don't forget to git add and git commit the file.


You will turn in your assignments using the
submission website. You need to request once an API key from the submission website before you can turn in any assignments or labs.

After committing your final changes to the lab, type make handin to submit your lab.

$ git commit -am "ready to submit my lab"
[util c2e3c8b] ready to submit my lab
 2 files changed, 18 insertions(+), 2 deletions(-)

$ make handin
tar: Removing leading `/' from member names
Get an API key for yourself by visiting
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 79258  100   239  100 79019    853   275k --:--:-- --:--:-- --:--:--  276k
make handin will store your API key in myapi.key. If you need to change your API key, just remove this file and let make handin generate it again (myapi.key must not include newline characters).

If you run make handin and you have either uncomitted changes or untracked files, you will see output similar to the following:

 M hello.c
?? bar.c
?? foo.pyc
Untracked files will not be handed in.  Continue? [y/N]
Inspect the above lines and make sure all files that your lab solution needs are tracked i.e. not listed in a line that begins with ??. You can cause git to track a new file that you create using git add filename.

If make handin does not work properly, try fixing the problem with the curl or Git commands. Or you can run make tarball. This will make a tar file for you, which you can then upload via our web interface.

Optional challenge exercises